Color measurement systems, especially for performing in-line color measurements in a multicolor printing press and, possibly, automatic adjustment and/or setting of inking units of the printing press are already known as such in the field of commercial printing. Such known systems are typically used in connection with commercial offset printing presses that are used to print various types of commercial products using the well-known four-color CMYK (Cyan-Magenta-Yellow-Key Black) subtractive color model, i.e. by printing multicolor patterns consisting of a combination of halftone raster patterns printed using the four primary colors Cyan, Magenta, Yellow and Black.
International application No. WO 2007/110317 A1 (and corresponding US publication No. US 2010/0116164 A1), which is incorporated herein by reference in its entirety, for instance discloses a method for adjusting an inking unit of a printing press. During a setup phase of the printing press, a small number of sheets are run through the printing press and the resulting printed sheets are inspected by means of a first measuring device (which is not integrated into the printing press), such as a densitometer, color spectrometer or a measuring instrument for combined densitometric and colorimetric measurements. The values measured by the first measuring device are compared to predetermined reference values and adjustments of the inking units of the printing press are made so that the values measured by the first measuring device match as closely as possible the desired reference values. A set of “first actual values” representative of the desired settings are thereby determined and stored as a result of the setup phase and the printing press can be released for production runs. At least a second measuring device is provided downstream of the printing units of the printing press in order to inspect the sheets during production, which second measuring device is installed in the printing press. Such second measuring device is for instance embodied as an in-line inspection system comprising at least one camera system and at least one illumination unit. The camera system is typically a color camera system comprising a line-scan sensor or an array sensor based on CCD or CMOS technology. The illumination unit typically comprises light-emitting diodes, or LEDs, or like illumination elements. The second measuring device records an image of at least one, preferably of all of the sheets which are printed on the printing press and converts the recorded images to digital image data which is fed to an image processing system as a set of “second actual values”. During a learning phase, the set of “second actual values” is measured and stored as reference values for controlling an adjustment unit which adjusts the inking units of the printing press. Upon completion of the learning phase, all further printed products which are produced on the printing press are evaluated on the basis of the reference values that were established during the learning phase and any deviation between the reference values and the measured values which exceeds an acceptable tolerance is corrected by means of the adjustment unit.
According to WO 2007/110317 A1, measurements are typically made on at least one measuring strip (or “color control strip”) that forms part of the patterns printed on the sheets, which measuring strip is typically located in a margin of the sheet, such as the margin at the leading edge of the sheet, outside the effective printed region of the sheet where the actual prints are carried out.
An example of such a measuring strip is disclosed in German patent application No. DE 10 2008 041 426 A1. This measuring strip comprises a plurality of juxtaposed color control fields, including color control fields printed in the primary colors (i.e. Cyan, Magenta, Yellow, Black colors), which color control fields are positioned in dependence of the relevant inking zones of the inking units of the printing press where ink adjustments are carried out.
European patent No. EP 0 142 469 B1 (and corresponding U.S. Pat. No. 4,660,159—see also EP 0 142 470 B1 and U.S. Pat. No. 4,665,496) discloses a method for adjusting an inking unit of a printing press. Reference reflectance values for a printed sheet are determined outside of the printing press by means of a scanning device, such as a plate scanner. Actual reflectance values of printed sheets which are being printed on the printing press are measured during production using a densitometer. The actual reflectance values and the reference reflectance values are compared with one another in a computer system. Based on the results of this comparison, control values for adjusting the inking units are calculated and ink feed elements are controlled on the basis of these control values. According to EP 0 142 469 B1, measurements are made directly in the printed image itself, the printed image being subdivided into a plurality of image elements whose reflectance values are measured. In this way, the use of special color measuring strips may be eliminated.
International application No. WO 2005/108083 A1 (and corresponding U.S. Pat. No. 7,515,267 B2) discloses a method for determining color and/or density values for monitoring and/or regulating a printing process in a printing apparatus, especially for use in a sheet-fed commercial offset printing press. According to WO 2005/108083 A1, measuring areas of a printed sheet are measured photoelectrically during the printing process and color and/or density values for the relevant measuring areas are determined. Deviations in the measured color and density values, as compared to measurements made outside of the printing press, are corrected.
International application No. WO 2005/108084 A1 (and corresponding U.S. Pat. No. 7,398,733 B2) discloses a method for in-line measurement of spectral, densitometric or color values measured on sheets being printed on a sheet-fed commercial offset printing press, which method involves a color calibration process. Measurements are made on a color control strip (see FIG. 9 of WO 2005/108084 A1) that is printed next to the effective printed region where the actual prints are carried out. Such color control strip comprises a plurality of juxtaposed color control fields, including control fields printed in the primary colors (i.e. Cyan, Magenta, Yellow, Black colors), which control fields are positioned in dependence of the relevant inking zones of the inking units.
U.S. Pat. No. 5,724,259 discloses a system and method for monitoring color in a commercial offset printing press. Measurements are made on a color bar (or “color control strip”—see in particular FIG. 5a of U.S. Pat. No. 5,724,259) comprising a plurality of juxtaposed color control fields printed in the primary colors (i.e. Cyan, Magenta, Yellow, Black colors) and with different tones (e.g. 100%, 75%. 50%, 25%) and combinations thereof including Blue (i.e. subtractive addition of Cyan and Magenta colors), Red (i.e. subtractive addition of Magenta and Yellow colors) and Green (i.e. subtractive addition of Cyan and Yellow colors).
European patent No. EP 0 394 681 B1 (and corresponding U.S. Pat. No. 5,023,812) discloses a method for controlling ink feed of a printing press wherein a sheet printed by the printing press is measured photoelectrically in a color control strip having a plurality of juxtaposed color-measuring fields, color measurement being carried out by a measuring head forming part of a densitometer or spectrometer, which measuring head scans the color control strip. A similar approach is disclosed in European patent No. EP 0 337 148 B1 (and corresponding U.S. Pat. No. 5,122,977).
European patent application No. EP 0 434 072 A2 also discloses color control strips for use in conventional four-color commercial offset printing. Further examples of color control strips or like color control elements are disclosed in European patent No. EP 0 590 282 B1, German patent publication DE 10 2007 029 211 A1 (see also corresponding US publication No. US 2008/0314268 A1), and U.S. Pat. No. 4,947,746.
All of the above known solutions are used for performing color measurements in commercial offset printing presses, i.e. printing presses of the type based on four-color composite printing using the CMYK subtractive color model. Printing presses of this type comprises at least four distinct printing towers which are each designed to print one of the four primary colors. Additional printing towers may be provided to print special colors and/or for the purpose of coating the printed substrates.
The above solutions are satisfactory as far as applications to commercial offset printing presses are concerned and basically require the use of a rather simple color control strip comprising a plurality of color control fields representative of the relevant primary colors that are printed (i.e. Cyan, Magenta, Yellow, Black) and, possibly, simple combinations thereof (e.g. Blue/Cyan+Magenta, Red/Magenta+Yellow, and Green/Cyan+Yellow) and/or additional special colors.
Commercial four-color offset printing is based on the printing of different raster patterns of each one of the four primary colors which are combined together to create, by subtractive color combination, a visual impression of various multicolor tones. In that respect, the design of the color control strip, and more precisely the locations of the relevant color control fields, bears no real importance, all of the relevant primary colors being typically distributed over the whole surface of the printed product.
The typical approach in terms of design of the relevant color control strips is to design those in dependence of the relevant ink zones where ink is applied and can be adjusted. The known color control strips therefore typically consist of a repetition, for each ink zone, of a predetermined succession of color control fields.
In contrast to commercial (offset) printing, security printing (as applied for instance for the production of banknotes) is not at all based on the use of a four-color printing process relying on the CMYK subtractive color model. Rather, solid patterns are printed using different printing inks of the desired colors (i.e. a blue pattern is printed using a blue printing ink, a brownish pattern using a brownish ink, a copper-like pattern using a copper-coloured printing ink, etc.).
Typical color control strips as used in commercial printing are not suitable for security printing applications for the purpose of measuring the printed colors, even less for the purpose of automatically controlling the ink supply. There is therefore a need for a new and improved solution which can suitably cope with the specific requirements of security printing.